Evaluation of the effects of fermentation of buffalo curd and acidity on survival kinetics of Listeria monocytogenes

Listeria monocytogenes, a food-borne pathogen often found in milk and milk products, causes listeriosis in pregnant women, newborns, elderly and immuno-suppressed people. There have been reports on survival of Listeria in various milk products in the world, but comparable studies on the survival of Lis-teria in buffalo curd have not been reported. Therefore, the present study investigated the effects of fer-menting buffalo milk and acidity on survival kinetics of L. monocytogenes. L. monocytogenes FSTLC2 and lactic acid starter cultures were aseptically introduced to boiled and cooled buffalo milk (fat 6%, protein 4%) and the mix was allowed to ferment at ambient temperature (26 ± 2^oC) for 18h. The Lis-teria count on Modified Oxford Agar (MOX; Oxoid Ltd.), lactic acid bacteria count on MRS Agar (Oxoid Ltd.), pH and titratable acidity were determined in the beginning and at 2h intervals during fer-mentation. The effect of pH/acidity on Listeria was determined by introducing Listeria to buffalo milk with pH values (pH 4.0, 4.5, 5.0, 5.5) adjusted using 88% lactic acid (BDH Chemicals) and enumerating Listeria on MOX Agar at 12h intervals for 96h. It was observed in the present study that Listeria count decreased over time and after 16h of fermentation of buffalo milk at ambient temperature, Listeria can-not be detected on MOX Agar. The pH value decreased from 6.8 to 4.1 and titratable acidity (lactic acid %) increased from 0% to 1.2 % during fermentation. Fermentation appeared to be an effective preser-vation technique in eliminating Listeria in buffalo curd. The pH value of 5.5 appeared to be the critical pH for inactivation of Listeria as no growth of Listeria was observed below pH 5.5. The total inactiva-tion of Listeria in buffalo milk appeared to be due to lowering of pH coupled with increasing titratable acidity as well as action of bacteriocins, especially Nisin, produced by lactic acid bacteria during fer-mentation. It is clear that fermentation is an effective tool in inactivating Listeria in buffalo milk.  

Keywords: Listeria monocytogenes, Buffalo curd, Fermentation, Inactivation, Acidity

DOI: http://dx.doi.org/10.4038/tare.v13i4.3292

TARE 2010; 13(4): 94-98